An inlet channel 1.7 meters above the bottom level of the 4.26 meter deep tank allowed excess water to escape. Other inlets prevented siltation of the tanks and erosion of the banks. After a ship would have unloaded its cargo, the gates would have opened and allowed it to return to the Arabian sea waters in the Gulf of Combay.

Archaeological finds from the excavations testify to trade with ancient Egypt and Mespotamia. The hydraulic knowledge of the ancient Harappans can be judged by the fact that boats could dock at Lothal in the 1850's. In 1942 timber was brought from Baruch to nearby Sagarwala. It is said that then the dockyard could hold 30 ships of 60 tons each or 60 ships of 30 tons each. This would be comparable to the modern docks at Vishakapatnam.

Right you are, The Imperial! And IVC seals have been unearthed in Mesopotamia, I believe. It is hardly plausible that peoples so near along a coast would not have traded extensively, and used boats for transport.

Mesopotamia and other bronze age cultures never developed trade to a comparable degree to the more recent classical cultures (i.e. greeks, romans, carthaginians, etc). They used small ships to trade luxury goods in small quantities.

However, Mesopotamia was integrated with Syria, Asia Minor, Greece and Egypt by around 1500 BC. Mesopotamia never integrated itself with the Indus valley culture, otherwise india wouldn't have developed as a separate civilization from the west. In the bronze age trade was mostly concentrated in the Eastern Mediterranean, with perhaps very small scale trade from the persian gulf to the indus valley.

"Trade with Mesopotamia has been noted, as Indus pottery has been discovered in the ancient city of Tell Asmar

A number of typical Indus seals have also been found in Sumer, seals dating back to between 2300 and 2000 BC. The finding of Indus seals in Mesopotamia suggests that people of Indus may have resided in this territory; possibly merchants who were keeping up a constant trade with the Mesopotamian people (cotton was a staple export of Indus, and could have been the crop that brought these two civilizations into contact)."

"According to Hawkes, the people of Indus sailed in "high-prowed, single masted" boats, sound for carrying the trafficked goods (Hawkes 1973:270). The excavated site at Lothal (another ancient city within the Indus Valley) has revealed harbour works, and the Harappan people may have been more advanced in their nautical skills than was originally perceived."

How much is essential. Since a small enough volume of trade is about the same as no trade at all. We would need evidence of continuous, significant trade between mesopotamia and the indus valley, not only a sample of finds of size 1.